/**
 * Defines a new constant.
 * Based on code by Hugo Giraudel, http://www.sitepoint.com/dealing-constants-sass/
 * @param  {String} $name  Name of new constant.
 * @param  {Misc}   $value Value of constant
 */
@mixin const($name, $value) {
  @if (map-has-key($CONSTANTS, $name)) {
    @error 'Constant `#{$name}` is already defined.';
  }
  $CONSTANTS: map-merge($CONSTANTS, ( $name: $value )) !global;
}